1. Field of the Invention
The present invention relates to a plate-like particle of cathode active material for lithium secondary battery, a cathode of a lithium secondary battery and a lithium secondary battery.
2. Description of the Related Art
A cathode active material for lithium secondary battery (may be referred to as a lithium ion secondary cell) using a plate-like particle of a lithium composite oxide (lithium transition metal oxide) having a layered rock salt structure is well-known (e.g., Japanese Patent Application Laid-Open (kokai) No. Hei 5-226004).
With respect to this kind of the plate-like particle, charge and discharge characteristics can be enhanced by means of exposure of the crystal plane through which lithium ions are intercalated and deintercalated (for example, the (104) plane) as much extent as possible to a plate surface. Also, filling ratio of cathode active material in a cathode can be raised and high capacity can be achieved by enlarging size (for example, to the extent of 5 μm or more and less than 30 μm in thickness) of the plate-like particle.
Meanwhile, impairment of capacity by repetition of charge-discharge cycles (may be referred to as impairment of cyclic characteristic) can occur in this kind of the lithium secondary battery. In order to elucidate the cause for the impairment, the cathode tested in an experiment example where the cyclic characteristic had been impaired were previously observed under an electron microscope. Through observation, cracks were found to be generated at the grain boundary in the plate-like particles (hereinafter, the cracks are referred to as “grain boundary cracks”). Furthermore, separation was found between a binder which contained an electroconducting aid such as carbon and the plate-like particles which were dispersed in the binder (hereinafter, the separation is referred to as “interface separation”).
The grain boundary crack and the interface separation are thought to be caused by crystal lattice expansion/contraction associated with intercalation and deintercalation of lithium ions in charge-discharge cycles, wherein the crystal lattice expansion/contraction includes the expansion/contraction of volume and the expansion/contraction not involving change in volume. Thus, such grain boundary crack or interface separation, which results in generation of an electrically isolated portion (i.e., a portion which does not contribute to capacity) through breakage of an electrical conduction path in a cathode active material layer, to thereby reduce capacity, is thought to be a possible cause for deterioration in cycle characteristic.